Detection of objects is important in many applications. For example, for security reasons, detection of objects is important to avoid allowing banned objects from entering and/or exiting a building, plane, or other location. Detection of objects can be difficult, however, if the detection is merely visual, as objects can easily be concealed from view. Thus, entrances to courthouses employ metal detectors to detect any sort of gun, knife of other typically metallic dangerous weapon.
Detection of metallic objects is not adequate for many security applications, as there are many objects that are undesirable that are not metallic whatsoever. And if visual inspection is not adequate to properly detect undesirable objects, then there is a need for a system that can accurately detect objects or else undesirable objects will remain undetected.
Detection of objects is also important for tracking or identification purposes wholly separate from security. It is desirable to correctly identify an object so that its movement can be accurately tracked, even when there is no viable means of placing a tracer, beacon, or other identification marking or code on the object. Thus, there is a need for a system that can detect an object so that the object's movement can be tracked without placing tracking means on the object.
Any object detection system relies on a property or characteristic of an object. For example, if the detection system is visual, the contour or the color of an object might be relied upon to detect certain objects. The specific density is one of many properties of an object, and remains static. Once an object's specific density is known, that object can be detected. However, the problem remains how to detect the specific density of an object if that object is not to be disturbed. Thus, there is a need for system for detecting the specific density of an object so that the object can be identified, wherein the object remains undisturbed.
The present invention deals with two high level concepts that are common in the art. The first concept is mode-hopping. Mode-hopping is an energy transfer from one transverse electronic mode (TEM) to another. For example, the most common mode hopping occurs between the fundamental mode (TEM00) and the donut mode (TEM01). Mode-hopping has been observed in semiconductor lasers due to the laser light fed back in to the laser cavity as a result of reflections. The laser energy distribution in a beam will switch from one mode to another as a function of the laser light fed back in to the cavity.
Inventions that discuss the mode-hopping phenomenon usually discuss the negative aspects. Mode-hopping is seen as a drawback to most lasers, particularly as it deals with an increase in temperature. Mode-hopping creates a situation by which, for a given pumping current, the laser can hop to a completely different mode. This “instability” has been linked to the occurrence of unwanted intensity noise, a change in injection strength (detuning), a reduction in beam power, and overall distress to users of various mechanisms utilizing lasers (including semiconductor lasers in compact disc players and bar-code scanners).
Mode-hopping has also been connected to problems other than use of lasers. In telecommunications, the switching from one mode to another affects the maximum data transmission rate, because different wavelengths have different velocities in single-mode fibers with high dispersion.
The second concept common in the art is the gravity meter, also known as a gradiometer. The concept of gradiometers has been known for some considerable time. Gradiometers measure the differential curvature or ellipticity of gravity equipotential surfaces, the rate of change of the increase of gravity in the horizontal direction, and/or the rate of increase of gravity in the vertical direction. Their object is to measure small changes in the acceleration of a mass due to gravity, known as “g”. Through discovery of “g”, one can determine the mass, specific density, etc. of a given space.
While gradiometers provide a method by which to obtain data regarding spaces, particularly dealing with land surveillance, most gradiometers have been expensive to manufacture and are unsuitable for long-term installation in the field. Because of the expense, care and accuracy that need to be put into their use, gradiometers are not suitable for everyday use. Furthermore, the gravity gradient measurements are associated with significant noise patterns.